1. Field of Invention
This invention relates to the conversion of chemical energy to electrical energy. In particular, the present invention relates to an electrode design having a first cathode active material of a relatively low energy density but of a relatively high rate capability and a second active material having a relatively high energy density but of a relatively low rate capability. The first and second active materials are short circuited to each other by contacting the opposite sides of spaced apart first and second current collectors, the second active material being at an intermediate position with the first active material contacting the opposite, outer current collector sides. A preferred form of the cell has the electrode as a cathode connected to a terminal lead insulated from the casing serving as the negative terminal for the anode electrode. The present electrode design is useful for powering an implantable medical device requiring a high rate discharge application.
2. Prior Art
As is well known by those skilled in the art, an implantable cardiac defibrillator is a device that requires a power source for a generally medium rate, constant resistance load component provided by circuits performing such functions as, for example, the heart sensing and pacing functions. From time-to-time, the cardiac defibrillator may require a generally high rate, pulse discharge load component that occurs, for example, during charging of a capacitor in the defibrillator for the purpose of delivering an electrical shock to the heart to treat tachyarrhythmia, the irregular, rapid heartbeats that can be fatal if left uncorrected.
It is generally recognized that for lithium cells, silver vanadium oxide (SVO) and, in particular, ε-phase silver vanadium oxide (AgV2O5.5), is preferred as the cathode active material. This active material has a theoretical volumetric capacity of 1.37 Ah/ml. By comparison, the theoretical volumetric capacity of CFx material (x=1.1) is 2.42 Ah/ml, which is 1.77 times that of e-phase silver vanadium oxide. For powering a cardiac defibrillator, SVO is preferred because it can deliver high current pulses or high energy within a short period of time. Although CFx has higher volumetric capacity, it cannot be used in medical devices requiring a high rate discharge application due to its low to medium rate of discharge capability.
A novel electrode construction using both a high rate active material, such as SVO, and a high energy density material, such as CFx, is described in U.S. Pat. Nos. 6,551,747 to Gan and 6,645,670 to Gan. These patents are assigned to the assignee of the present invention and incorporated herein by reference. FIG. 1 is a schematic view of a portion of a cathode electrode 10 according to the Gan patents. Electrode 10 comprises spaced apart current collectors 12 and 14 supporting layers 16 and 18 of a first cathode active material on their respective outer major sides 12A and 14A. The first cathode active materials 16, 18 are of a relatively high rate capability, but of a low energy density (for example, SVO) in comparison to a second cathode active material 20 of a relatively high energy density, but a low rate capability (for example, CFx) sandwiched between and in contact with the inner major sides 12B and 14B of the respective current collectors 12, 14. The current collectors 12, 14 are shown as perforated structures having a carbonaceous material 22 contacting at least the side facing the CFx material.
The Gan '747 patent further teaches at column 10, lines 29 to 58 that:                “the high volumetric capacity CFx active material is quantitatively converted into or used as high power energy of the SVO material. It is believed that during high energy pulsing, all the discharge energy is provided by the SVO material. Above the discharge voltage of the CFx electrode material, only SVO electrode material is discharged with the SVO material providing all of the discharge energy for pulsing as well as for any background load discharging. Under these discharge conditions, the CFx active material is polarized with respect to the SVO material discharge voltages. Then, when the lithium cells having the sandwich cathodes of the present invention are discharged to the working voltage of the CFx material, both the SVO and CFx active materials provide the energy for background load discharging. However, only the SVO material provides energy for high rate pulse discharging. After the SVO active material is pulse discharged, the potential of SVO material tends to drop due to the loss of capacity. When the SVO background voltage drops below the working voltage of the CFx material, the SVO material is believed to be charged by the CFx material to bring the discharge voltage of the sandwich cathode materials to an equal value. Therefore, it is believed that the SVO material acts as a rechargeable electrode while at the same time the CFx material acts as a charger or energy reservoir. As a result, both active materials reach end of service life at the same time.”        
In that respect, both Gan patents relate to a lithium electrochemical cell where the SVO material is charged by the CFx material when the formers background voltage falls below that of the CFx material to bring the discharge voltage of the disparate cathode materials to an equal value. The SVO material acts as a rechargeable electrode while the CFx material is simultaneously acting as a charger or energy reserve. So, in some sense both Gan patents teach a lithium cell having a cathode with internal re-charge characteristics.
In a typical lithium/silver vanadium oxide cell (Li/SVO) powering an implantable medical device, the cathode current collector supports two layers of SVO contacted to each of its opposed major sides. A typical cathode current collector is of titanium being about 0.003 inches thick. This provides the cathode with sufficient current carrying capability for both the relatively low rate discharge and, more importantly, for the high rate, pulse discharge. However, if the current collectors 12 and 14 (FIG. 1) in the sandwich cathode design described in the previously discussed Gan patents were of a thickness similar to that of a typical Li/SVO cell, the total current collector thickness would be twice as large as the conventional cell. This would detract from the many desirable attributes inherent in the Gan cell constructions including providing ample high pulsing capability for powering modern implantable cardiac defibrillators, and the like, with little or no voltage delay.